The present invention relates to fiber-reinforced composites. More particularly, the present invention relates to fiber-reinforced composites having a gradient in thermal expansion coefficient across at least one dimension of the component.
Fiber reinforced ceramic matrix composites having high temperature applications requiring thermal and environmental stability and good thermal shock resistance are increasingly used for combustion and exhaust components in jet and rocket engines, ceramic burner inserts, heat exchanger tubes, and the like.
The combustion and exhaust components, in order to serve their intended purpose, have to be operated at high temperatures and under mechanical and thermal stresses. In some cases, thermal stresses result from a temperature gradient through the thickness of the composite when one surface of the composite, serving as the component, sees a much high temperature than the other surface. The hot side of the component may be in compression and would benefit from reduced thermal expansion. The cold surface of the component may be in tension and would benefit from higher thermal expansion. It has been observed that prior art fiber-reinforced ceramic matrices often microcracked due to these stresses, thereby leading to the failure of these composites used as combustion and exhaust components.
Continuous oxide fibers such as Nextel 610 and Nextel 720, both available from 3M Company, Minneapolis Minn., have been used in porous matrices of alumina and silica to achieve damage-tolerant, high temperature composite materials for use in oxidizing conditions. Oxide-oxide composites are more resistant to environmental effects, such as oxygen, water and salts, than are non-oxide composites, e.g., SiC- or Si.sub.3 N.sub.4 -based composites. However, oxide-oxide composites typically have higher thermal expansion coefficients and lower thermal conductivity than, for example, SiC. The thermal conductivity is further lowered by the porosity levels that are often employed in the matrices of such composites to ensure damage tolerance. In applications such as combustion liners and engine nozzles, the large thermal gradients and the high thermal expansion result in the development of large stresses. Any reduction in these thermal stresses will greatly enhance the durability and applicability, permitting wider use of these composites.
Other fiber-matrix composites, such as carbon-carbon composites as well as SiC- or Si.sub.3 N.sub.4 -based composites, may also exhibit this gradient in thermal expansion coefficient across the thickness of the component when employed in high temperature applications.
Accordingly, it is an object of the present invention to provide improved fiber-matrix composites for use in high temperature applications which impose large thermal gradients.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.